Gas turbine engines generally operate on the principle of compressing air within a compressor section of the engine, and then delivering the compressed air to the combustion section of the engine where fuel is added to the air and ignited. Afterwards, the resulting combustion mixture is delivered to the turbine section of the engine, where a portion of the energy generated by the combustion process is extracted by a turbine to drive the engine compressor. High bypass turbofan engines are widely used for high performance aircraft which operate at subsonic speeds. High bypass turbofan engines have a large fan placed at the front of the engine which serves to produce greater thrust and reduce specific fuel consumption. The fan serves to compress incoming air, a portion of which is then delivered to the combustion chamber, though a larger portion is bypassed to the rear of the engine to generate additional engine thrust.
The fan is contained within a fan case equipped with a shroud, such that the shroud circumscribes the fan and is adjacent to the tips of the fan blades. The shroud serves to channel incoming air through the fan so as to ensure that the bulk of the air entering the engine will be compressed by the fan. However, a small portion of the air is able to bypass the fan blades through a radial gap present between the fan blade tips and the shroud. Because the air compressed by the fan blades is used to generate thrust and feed the turbine section of the engine, engine efficiency can be increased by limiting the amount of air which is able to bypass the fan blades through this gap. Accordingly, the fan and shroud are manufactured to close tolerances in order to minimize the gap.
However, manufacturing tolerances, differing rates of thermal expansion and dynamic effects limit the extent to which this gap can be reduced. Furthermore, during the normal operation of an aircraft turbofan engine, the fan blades may rub the shroud as a result of a hard landing or a hard maneuver of the aircraft. Any rubbing contact between the fan blade tips and the shroud will abrade the tips of the rotors, tending to further increase the gap between the shroud and blade tips, thereby reducing engine efficiency. Accordingly, it is well known in the art to cover the portion of the shroud adjacent the blade tips with an abradable material, such that the abradable material will sacrificially abrade away when rubbed by the fan blades. Inherently, as the abradable material is removed, the gap between the blade tips and the surface of the abradable material will increase, necessitating that the abradable material be restored in order to maintain desirable aerodynamic efficiencies associated with a smooth abradable surface and a small gap between the abradable surface and the fan blades.
Various materials and processes have been suggested to form and restore the abradable surface. A common method of restoring the abradable surface is to completely machine out the old abradable material and bond a panel formed of the abradable material in its place. However, this process is time consuming and expensive. The shroud must be disassembled from the engine and placed on a turning machine to remove the old abradable material. Bonding new material in its place typically requires an oven in which the adhesive used to adhere the new material to the shroud is cured. The above requires a large maintenance facility to which at least the front of the engine must be transported for disassembly. Due to the special equipment required to perform these machining and curing operations, a limited number of facilities are available for restoring the abradable material. As a result, additional costs, scheduling and transport problems are common.
Accordingly, it would be advantageous to provide an abradable shroud assembly whose abradable material can be readily restored without requiring the entire engine to be disassembled, such that restoration can be performed in the field. In addition, such an assembly would preferably utilize existing hardware and materials so as to be compatible with turbofan engines currently in service.